The present invention relates, in general, to integrated circuit test systems, and more particularly, to integrated circuit test systems for semiconductor die.
As pad densities increase on semiconductor die, automated testing of the die becomes increasingly difficult. The function of any interconnect system, whether a probe card, test socket or burn-in socket, is to provide a reliable interconnect between the individual die and the IC tester. In the past, traditional wafer probe cards were generally "Wire Needle Epoxy Ring" or "Ceramic Blade" styles. These probe cards have tips that touch the die pads at their needle ends to give an electrical connection via the test head pogo pins to the tester lines. These prior art methods using probe needles to contact the pads have proved insufficient for high pad density devices. The industry has turned to flexible circuit overlays having bare trace end points aligned to the pads of the semiconductor die. However, the conventional flexible membrane die probe technologies have certain failings as well.
One of these failings is that conventional membrane probe cards typically have a large surface area near the wafer during probing. This area often drapes or droops into contact with wafer areas adjacent to the die under test which causes cleanliness problems. Airborne contamination, manual handling debris and processing residue all contribute to contamination of membrane test surfaces.
Membrane probe cards dislodge wafer process particles which needle probe cards previously did not touch. These particles occur in the streets between dice or on the surface of most wafers as a result of the wafer fabrication processes. Surface contamination of membrane probe cards is a significant issue made more severe by any large membrane surface area near the wafer acting as a "dust mop" which collects dislodged particles. Membrane probe cards will collect debris from areas not directly involved in die testing, such as the process control areas of the wafer. In the past, needle probe cards were not plagued with this problem because their contact area was limited to the needle tips touching on the die contact pads.
Another problem with conventional membrane probe cards arises because they employ an elastomer to press the membrane against the wafer to provide compliance for the bumps on the membrane. The electrical traces or bumps on the membrane tend to bury themselves in the elastomer, in a phenomenon referred to as the pillow effect, as the membrane is pressed into contact with the wafer surface. The maximum load, at each of the contact points between membrane traces and die pads, becomes somewhat limited as the trace is buried below the plane of the surface of the elastomeric material. Beyond this point, only a small fraction of additional force is transferred to the connection between trace and die pads. Therefore, it is difficult to get sufficient force at the contact points of conventional membrane probe cards to make reliable electrical connections.
In addition, membrane probe cards require a connection between the membrane circuitry and the printed wiring board interfacing to the electronic tester. Conventional membrane probe cards require precise positioning of the membrane in the proper location relative to the printed circuit card. Locating the flexible membrane is difficult due to the inherent problems associated with matching multiple radial patterns. Multiple circuit connections must then be made either by wire attachment, direct bonding or compression contact. Typically, membrane probe cards are only factory repairable at considerable cost because of the precise and delicate nature of a membrane replacement.
Consequently, what is needed is an integrated circuit test system which: 1.) does not exhibit the "dust mop" effect of picking up contaminate particles, 2.) does not exhibit poor electrical connections at the wafer because of the "pillow effect", and 3.) is not difficult and expensive to repair.